Method of processing alpha-beta titanium alloy

ABSTRACT

A PROCEDURE FOR IMPROVING THE DUCTILITY AND ROLLABILITY OF AN ALPHA-BETA TITANIUM, SUCH AS TI-6A1-4V, WHOSE INITIAL HOT REDUCTION OCCURS IN PART BELOW THE BETA TRANSUS TEMPERATURE. THE FURTHER AND FINAL PROCESSING STEPS ARE CHARACTERIZED BY A SEQUENCE OF OEPRATIONS WHICH INCLUDE AT LEAST TWO ROLLING STAGES TO FINAL GAUGE. THE SEQUENCE IS FURTHER CHARACTERIZED BY THE PROVISION OF A LOW TEMPERATURE INTERMIEDIATE ANNEAL SEPARATING EACH SAID ROLLING STAGE, AND THAT AT LEAST ONE OF SAID ROLLING STAGES REDUCES THE ALLOY STRIP BY AN AMOUNT GREATER THAN 25% AT A TEMPERATURE BETWEEN ABOUT 430-790*C. (800-1450*F.)

March 14, 1972 CHALK 3,649,374

METHOD OF PROCESSING ALPHABETA TITANIUM ALLOY Filed April 24, 1970 2 Sheets-Sheet 1 x5 JOOO TEMPEQA TL/PE o C P. I v A 77 V/5x50 AMOUNT (4/v0 //VC,QEA5/N6/5 STAB/L /ZEE Fig.1

lNVENTOR/S DA \//0 Z. CHALK ATTORNEYS March 14, 1972 LK 3,649,374

METHOD OF PROCESSING ALPHA-BETA TITANIUM ALLOY Filed April 24, 1970 2 Sheets-Sheet 2 fifiw/vsus 1000 LU 400- E 02.

Fig, 2 INVENTOR'S DAV/D L. CHALK BY .%a4m, 50% fill/W710 ATTORNEYS United States Patent O 3,649,374 METHOD OF PROCESSING ALPHA-BETA TITANIUM ALLOY David L. Chalk, Monroe, Ohio, assignor to Armco Steel Corporation, Middletown, Ohio Filed Apr. 24, 1970, Ser. No. 31,529 Int. Cl. C22f 1/18 US. Cl. 14811.5 F 11 Claims ABSTRACT OF THE DISCLOSURE A procedure for improving the ductility and rollability of an alpha-beta titanium, such as Ti6Al4V, whose initial hot reduction occurs in part below the beta transus temperature. The further and final processing steps are characterized by a sequence of operations which include at least two rolling stages to final gauge. The sequence is further characterized by the provision of a low temperature intermediate anneal separating each said rolling stage, and that at least one of said rolling stages reduces the alloy strip by an amount greater than 25% at a temperature between about 430-790 C. (2300-1450 F.).

BACKGROUND OF THE INVENTION This invention relates to a procedure for improving the ductility and rollability of an alpha-beta titanium alloy without sacrifice to the high strength values associated with these types of alloys.

At the outset, the mixed phase, or alpha-beta alloys, should be distinguished from the commercially pure titanium, the alpha stabilized alloys, and the all beta stabilized alloys. Commercially pure titanium is characterized by the presence of two distinct microstructures with changes in temperatures. For example, at temperatures above 885 C. (1620 F.), the titanium assumes a crystalline structure identified as body-centered cubic and this has become known as the beta phase. Below this temperature, a closepacked hexagonal structure developes and this is known as the alpha phase.

Through experience in the development of the titanium alloys, it has been shown that numerous elements may be added to the titanium to afiect changes in properties. These elements may be classified into two basic categories, namely, those alloys which tend to stabilize the alpha phase, and those alloys which tend to stabilize the beta phase. Thus, by adding one or more elements from each of the respective groups of elements, it is possible to produce what has now come to be known as the mixed phase or alpha-beta phase alloys. A typical alloy which falls within this description is one nominally containing 6% by weight aluminum, 4% by weight vanadium, and the balance substantially titanium. In this particular alloy, the aluminum acts as the alpha stabilizer, while vanadium is the beta stabilizer.

While all of the above named types of alloys have certain desirable characteristics as well as limitations, the alpha-beta alloys of titanium are somewhat unique in that they combine many of the desirable properties of both the alpha alloys and the beta alloys. However, they do have certain limitations which must be considered in fabricating and designing structures from this material.

In this latter regard, one of the difiiculties encountered by the producers of this material, particularly as it relates to the production of strip, was in the attainment of sufficiently high mechanical properties. For example, a typical specification covering this grade of material is as follows:

tensile strength: 965 MN/m. (140K s.i.) yield strength: 862 MN/m. (125K s.i.) elongation in 5.08 cm. (2"): 10% and radius transverse bend: 5T.

3,549,374 Patented Mar. 14, 1972 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 represents a typical constitution diagram of a titanium alloy showing the alpha, beta, and alpha-beta phase regions from room to elevated temperatures.

FIG. 2 is a fiow diagram of the processing sequence of this invention on an alpha-beta alloy of titanium.

BRIEF SUMMARY OF THE INVENTION This invention relates to a procedure for improving the ductility of the alpha-beta alloys of titanium, while maintaining the high strength thereof. This result is achieved by conducting at least a portion of the initial hot reduction below the beta transus temperature. In this rgeard, it was found that in the all beta region a structure which is hot worked therein will develop an accicular structure which is difiicult to remove or break up with further processing. However, by conducting a portion of the reduction, preferably at least 50%, below the beta transus it was found that this structure could be broken up to produce an alloy having an equiaxed structure.

It was further discovered that in utilizing certain processing controls, the later reduction and annealing sequence could be simplified to further enhance the mechanical properties. That is, improved ductility resulted when the processing sequence included at least two rolling stages having a low temperature intermediate anneal, one below about 820 C. (1500 F.) proceeding each rolling stage. In addition, when at least one of these rolling stages was conducted at a temperature between about 430790 C. (800-1450 F.), it was possible to utilize materially larger reductions than normally possible under relatively cold rolling conditions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT As stated previously, this invention is concerned with the production of high mechanical properties in the alphabeta alloys of titanium by an improved procedure For convenience in describing the preferred embodiment of this invention, reference may be made to FIG. 1 which shows a typical construction phase diagram of a titanium alloy. However, it must be made clear that in characterizing a titanium alloy by its crystalline structure, the characterization relates solely to the room temperature structure present in the alloy. Thus, even though the alloy will undergo phase changes with increasing temperatures, it will again revert to the basic or original crystalline structure by which it is characterized.

It should also be apparent from FIG. 1 that the character of a titanium base alloy can be drastically changed by merely adding to or removing alpha or beta promoting elements. The present invention therefore covers those alloys which contain sufficient amounts of one or more alpha promoting elements selected from the group consisting of aluminum, tin and antimony, and one or more beta promoting elements selected from the group consisting of vanadium, molybdenum, columbium, tantalum, chromium, manganese, and iron, to give the resulting alloy an alpha plus beta crystalline structure at room temperature. A commercial alloy which falls within this category is one nominally containing 6% by weight aluminum, and 4% by weight vanadium. The general chemistry for such an alloy is as follows: aluminum: 5.5-6.75% vanadium: 3.5-4.5%

nitrogen: 0.7% max. carbon: 110% max. hydrogen: 015% max. iron: .40% max. oxygen: 30% max. titanium: Balance While the invention should not be limited to this specific alloy, for convenience in understanding this invention, the further description will be directed thereto.

In regard to the phase changes which a titanium alloy undergoes upon heating and/or cooling, a phenomenon was discovered in the cooling of the beta phase. To assist in describing this phenomenon, reference is again made to FIG. 1, more specifically, the vertical line A-B, which designates the chemistry of a specific alloy. If it is assumed for said alloy that the alpha phase is present in an amount, X% by volume, the beta phase will be present in an amount 100 minus X% by volume. As the temperature of the alloy is raised from A to B, it will be observed that X decreases such that when the temperature reaches B, X will be substantially 0, and the alloy fully transformed to beta. The converse is true on cooling from B to A.

The constitution diagram, FIG. 1, has been modified by the inclusion of the line designated M It has been observed that in alloys of the type described herein, alphaprime [martensitic alpha] is formed by rapidly cooling through and below the M With such rapid cooling, the beta transforms to an unstable alpha which is supersaturated with the beta stabilizing element. The M represents the maximum temperature at which alpha-prime begins to form on cooling of the beta phase.

In the processing cycle of the present invention the M temperature has significance for the intermediate annealing steps in eventually securing optimum ductility properties.

With the foregoing as background, a discussion of the processing sequence is now oifered. Initially, an ingot of the alpha-beta alloy of titanium is hot reduced, such as by forging or rolling, down to a slab thickness on the order of about -18 cm. (4-7 inches). Typically, the hotter the alloy, the easier it is to reduce it. However, in the present invention, a portion of this initial reduction, preferably at least about the final 50%, must be conducted at temperatures below the beta transus. The latter is the temperature above which the alloy is totally in the beta phase.

It was discovered that when an alloy of the type described herein is worked in the all beta region, an accicular structure will develop. Such a structure is quite difficult to remove or break up with further processing. However, by conducting at least a portion of this initial reduction in the alpha plus beta region, it was found that this accicular structure could be broken up to produce an alloy having an equiaxed structure.

With the alloy in slab form, it is reheated to a temperature below the beta transus and reduced to strip in the form of a coil. Prior to any further reductions, the coil is subjected to a low temperature anneal below the M temperature. Preferably this is conducted at a temperature within the range of about the M down to about 75 C. (170 F.) below the M Optionally, this final initial low temperature anneal may be preceded by a beta anneal. This is suggested for those applications of the finally processed strip where controlled directionality or a minimum anisotropy of mechanical properties is desired. In other words, it is concerned with minimizing the difference between the respective yield and tensile strength between the longitudinal and transverse directions of rolling. While a brittle accicular structure may develop, it has been found that the subsequent low temperature anneal will restore sufficient ductility to the alloy to permit further processing.

In the hot reduced and annealed condition, the strip,

4 which is on the order of about 5 mm. (.20 in.) may be reduced to a final gauge of about 1.5 mm. (.06 in.) in a sequence including at least two reduction stages with an intermediate anneal therebetween.

Experience has shown that on alpha-beta alloys of titanium it was virtually impossible to reduce the strip in a single stage by amount greater than 25% without developing edge cracks, surface checks, or internal fissures, when such reductions were conducted at relatively cold temperatures, i.e., room temperature. However, when the rolling temperature was raised to the range of about 430 C. (800 F.) up to the M temperature, preferably above about 535 C. (990 1 it was found that much greater reductions could be taken without adversely affecting the strip. An additional advantage, heretofore not recognized in the prior art, became apparent from the warm rolling.

An oxidized titanium product has a scaled surface and an underlying oxygen enriched stabilized alpha layer, both of which are brittle. However, at temperatures above about 650 C. (1200 F.), the surface layers are ductile and can be rolled along with the underlying strip without initiating cracks. Thus, it is unnecessary to descale the strip prior to the warm rolling above said temperature.

Before conducting a further and/or final reduction, the reduced strip is subjected to a low temperature anneal below the M temperature. Preferably this is conducted at a temperature within the range of about the M, down to about C. (170 F.) below the M Where additional reduction stages are needed, after at least one warm reduction at 430 C. or higher, followed by the low temperature anneal, it was discovered that a cold reduction (little or no heat applied to alloy strip) up to about 45% was practical without developing the objectional defects noted above.

After the strip has been reduced to final gauge, it is then subjected to a final anneal at a temperature within the range of about 790 (1450") to about 950 C. (1740 B).

As an additional means to understand the present invention, a specific example of the processing sequence hereinbefore described is given below:

(1) Forge ingot to slab thickness of 10 cm. (4 in.) with final 50% reduction below beta transus temperature. (2) Hot roll from 955 C. (1750 F.) to 5.4 mm. (.21

in.) x 117 cm. (45 in.) coil.

(3) Anneal at 790 C. (1450 F.).

(4) Warm roll from 540 C. (1000 F.) to 2.67 mm.

(.105 in.) x 97 cm. (38 in.) coil.

(5) Anneal at 790 C. (1450 F.).

(6) Warm roll from 540 C. (1000 F.) to 1.83 mm.

(.072 in.) X cm. (37 in.) coil.

(7) Anneal at 790 C. (1450 F.).

(8) Cold roll from 1.83 mm. (.072 in.) to 1.52 mm.

(9) Final anneal at 845 C. (1550 PI).

When processed in the foregoing manner, a coil of Ti- 6Al-4V was found to possess the following properties:

A second coil of Ti6Al4V was processed in a similar manner except that the annealed strip of step (5) was cold reduced 43% from 2.67 mm. (.105 in.) to a final thickness of 1.52 mm. (.061 in.), and finally annealed at 845 C. (1550 F.). The following properties were ob- 5. A process for improving the ductility of an alphaserved: beta titanium alloy comprising the steps of reducing an ingot of said alloy to slab thickness such that at least a Ldirection Tdirection portion of said reduction is conducted at a temperature Yield Strength 918 MN/HLZ (134K 1,138 MN/m) (165 K below the beta transus, hot reducing said slab at a tem- 5.1. o si. n perature below the beta transus, annealing said hot re- Tensle Strength g% K 'g f duced alloy at a temperature below the M temperature, Perccntelong ation in 12. subjecting said annealed alloy to a rolling sequence 1n 'f 3f3 Passed Passed a plurality of stages, one of said stages being conducted 0 at a temperature about 430790 C. by an amount greater than 25%, and subjecting said alloy to a low temperature The foregoing represents eieafiy the attainment of good anneal below the M temperature intermediate adjacent mechanical properties without relying upon an elaborate l i Stagw and Complex PIeeeSSihg Sequence In feet, the latter alloy 6. The process claimed in claim 5 wherein said slab was Processed With a minimum of telling 0PeIati0I1Sah reduction is conducted at a temperature between about obvious savings in operating time and expense. Finally, 79 95 C it will be observed that these alloys were processed with- 7, Th process l i d i l i 6 wher in said last Out regard to eohtfoiiihg directionality of p p It named rolling stage is conducted at a temperature less such were desired, a beta anneal could have been included h 650 C, between Steps and Even y the inclusion of such 8. The process claimed in claim 7 wherein said rolling a p, a relatively Simple Procedure is taught to Produce stage is conducted at a temperature of at least 535 C. good mechanical Properties in alpha-beta alloys of 9. The process claimed in claim 6 wherein said titanium tanium. alloy consists essentially of about 5.56.75% by weight Since modifications y become pp to those aluminum, about 35-45% by weight vanadium, and the skilled in the art upon reading this specification, no limitab l ti ll tit ni m, tion is intended to be imposed herein except as set forth 10. Th process l i ed i laim 9 wherein aid hot in the appended claims. reduction below the beta transus results in said alloy The em dim nts f th invention n Which an eXehlhaving an oxidized surface layer and an underlying oxygen Sive P p y Privilege is claimed at e defined as follows! enriched stabilized alpha titanium layer, and that said 111 a PieceSS for improving the ductility and F011 last named rolling stage takes place without removal of ability of an alpha-beta titanium alloy which includes hot id idi d surface layer, reducing said alloy in strip form to an intermediate thick- 11 Th process l i d i l i 10 herein aid Hess, the improvement Comprising in Combination thereoxidized surface layer is removed subsequent said rolling with, the steps of subjecting said alloy strip to at least two rolling stages to final gauge, each said rolling stage separated from another by a low temperature intermediate anneal, whereby at least one of said rolling stages reduces References Cited said alloy strip by an amount greater than 25% at a temperature between about 430-790 C., and annealing UNITED STATES PATENTS stage, and that the finally reduced strip is annealed at a temperature about 760820 C.

fi an d ed 11 t t t t b t 2,857,269 10/1958 Vordahl 148-12.; iz g f g l g a W Snp a a empera me e We 2,950,191 8/1960 Vordahl 14811.5 F

2. The process claimed in claim 1 wherein said last 2,974,076 3/1961 Vordahl 148-127 named rolling stage is conducted at a temperature less 3,169,085 2/1965 Newman 148 11'5 F than about c 3,409,428 11/1968 Parris -175.5

3. The process claimed in claim 2 wherein said rolling stage is conducted at a temperature of at least 535 C. 45 DEWAYNE RUTLEDGE Pnmary Examiner 4. The process claimed in claim 1 wherein said titanium W. W. STALLARD, Assistant Examiner alloy consists essentially of about 5.56.75% by weight aluminum, about 3.5-4.5 by weight vanadium, and the X- balance essentially titanium. l48-12.7 

